1. Field of the Invention
The invention concerns a process for the preparation of flexible polyurethane foams having a good resistance to hydrolysis, improved damping with equivalent or improved physical properties, and high sound absorption capacity. The polyurethane foams are prepared by reacting organic polyisocyanates with mixtures of
(a) liquid polyester polyols produced by the polycondensation of organic dicarboxylic acids and a polyol mixture containing 1,4 butanediol, 1,5 pentanediol, 1,6 hexanediol, and at least one triol, and PA0 (b) di- to tetra-functional polyether polyols having molecular weights of 1500 to 6000,
in the presence of catalysts and blowing agents as well as optionally chain extenders, auxiliaries, and additives.
Selected polyester-polyether polyol mixtures which are employed in the process of this invention exhibit improved compatibility of the components and storage stability.
2. Description of the Prior Art
The preparation of polyurethane flexible foams is essentially known and is described in detail in the appropriate literature. The plastics handbook Volume VII "Polyurethanes" by R. Vieweg, A. Hoechtlen, Carl Hanser Publishers, Munich, 1966 is such an example.
In addition to catalysts, blowing agents, auxiliaries, and additives, toluene diisocyanates are used for the preparation of polyurethane flexible foams. In consideration of toxicity and reactivity, mixtures of toluene diisocyanates and crude diphenylmethane diisocyanates may be employed.
Preferably used as the polyhydroxyl compounds are polyether polyols particularly those based on ethylene and/or propylene oxide. One drawback of the polyurethane flexible foams produced from these polyols, is that the general physical properties do not meet the requirements in all areas of application.
If specific requirements are made of the physical property level of polyurethane flexible foams for definite areas of application, polyester polyols may be used for their preparation instead of the polyether polyols. However, the resultant foams have a low resistance to hydrolysis.
In order to reduce these drawbacks, German published Application No. 28 15 540 suggests the selection of certain starting components for the preparation of polyurethane flexible foams and the use of a urethane group containing mixture of diphenylmethane diisocyanates and polymethylene polyphenylene-polyisocyanates containing 55 to 85% by weight of diphenyl diisocyanates isomers as the polyisocyanates, polyester polyols or mixtures of polyester polyols and polyether polyols with a polyester polyether content of at least 60% by weight as polyhydroxyl compounds and water and mixtures of water or low boiling, optionally halogen containing hydrocarbons as the blowing agents.
Specific mention is made of polyester polyols having a molecular weight of 750 to 5000 and a functionality of 2 to 3.5 which are produced by the condensation of a dicarboxylic acid mixture consisting of succinic, glutaric, and adipic acids in certain quantity ratios and di- and trifunctional alcohols. In this manner polyurethane flexible foams can be obtained which have a high load bearing and energy absorption capacity as well as considerable resistance to hydrolysis.
However, even if this process is employed certain difficulties can occur in processing the polyester-polyether polyol mixtures, since the polyester polyols and polyether polyols described require correct mixing in order to avoid phase separation. This chemical behavior is not surprising since it is known from the literature that polyester polyols and polyether polyols for polyurethane flexible foams are not miscible (plastics handbook Volume VII, "Polyurethane" by R. Vieweg and A. Hoechtlen, Carl Hanser Publisher, Munich, 1966, Page 52).
Even small amounts of polyester polyol in polyether polyol and vice versa can disturb the processability of one or the other, resulting in cracks, voids, and cell disturbances in the resulting foam which in extreme cases may even result in a collapse of the foam. Tanks, lines, and foaming equipment must be cleansed extremely thoroughly when changing from polyester polyol to polyether polyols and vice versa.
Another drawback of commercially available polyester polyols is the low resistance to hydrolysis which is not improved by being incorporated in the polyurethane foam structure. The purpose of this invention is to produced flexible polyurethane foams with good resistance to hydrolysis and improved mechanical properties and particularly with a high compression strength and favorable damping behavior. Surprisingly, this can be accomplished by using mixtures of specific polyester polyols and polyether polyols as the polyhydroxyl compounds in the preparation of the flexible polyurethane foams.